Thailand's junta chief on Monday dismissed rumours of an impending coup as "fake news", as speculation ricocheted across a kingdom unsettled by the ill-fated political union between a princess and a party allied to the powerful Shinawatra clan.

Thai Prime Minister General Prayut Chan-O-Cha, pictured here in November 2018, dismissed rumours of an impending coup as 'fake news'

Conjecture has coursed through Thailand since Friday when the Thai Raksa Chart party proposed Princess Ubolratana, King Maha Vajiralongkorn's elder sister, as a candidate for premier after the March 24 election.

Hours later, a royal command from the king appeared to put a pin in her unprecedented political aspirations.

It said the monarchy was above politics and described his sister's candidacy as "highly inappropriate".

The slapdown by an unassailable monarch - protected by some of the world's harshest royal defamation laws - who has never addressed the public in such strong terms, set off a chain reaction.

A chastened Thai Raksa Chart, a key pillar in the election strategy of billionaire ex-premier Thaksin Shinawatra, swiftly agreed to comply with the command.

Election authorities meeting Monday are expected to discuss whether the use of the princess's name was unconstitutional, a first step towards dissolving the party.

Adding to the uncertainty, chatter of an impending coup against the ruling junta leader Prayut Chan-O-Cha and a major change in army top brass has billowed out, with the hashtag #coup trending in the top 10 in Thai Twitter.

But on Monday the gruff former general, who masterminded a putsch against the government of Yingluck Shinawatra, Thaksin's sister, in 2014, tried to stop it short.

"Rumours...? We're investigating. Fake news," he told reporters at Government House about the merits of the speculation.

- Coups and plots -

Thailand's generals have a penchant for coups, backroom plotting and factional struggles.

They have grabbed power 12 times since the end of absolute monarchy in 1932, including against existing juntas seen to have over-stepped their mark.

Prayut, a gruff ex-army chief turned junta head, has agreed to stand for premier after the election and is aided by an army-scripted constitution.

But critics say he has personalised power and outstayed his welcome with a public wearied by his finger-jabbing style.

The king appointed a new army chief, Apirat Kongsompong, last year from a rival faction of the army to Prayut and his junta allies.

Recent days have seeded unease, with the first election in eight years now seemingly dependent on behind-the-scenes power plays by the elite.

"Pls#NoMoreCoup WTF with this country," said one Twitter user, while another said "I wish we have only #election2019".

Meanwhile, the fate of Thai Raksa Chart hangs in the balance.

The party, a second to the Thaksin political powerhouse Pheu Thai, was expected to help the Shinawatra machine secure a majority in the 350-seat lower house.

But it is under intense pressure following its bid to bring in the princess.

"I think the party leader and board should take a responsibility by resigning," said Srisuwan Janya of the Association for the Protection of the Constitution, a royalist activist group, who submitted a petition to election authorities Monday calling for the party's censure.